Ummmm no, not always, Compression only needs to be enough to cause ignition via the heat produced. Any more than this actually reduces power output, as much of the developed power is used to compress the next power stroke. My 3.5hp Lister is very much more capable when running with the COV out, and will struggle with the same fixed load when the COV is wound in. Unlike a petrol engine, the cylinder pressure is largely constant over the power stroke as fuel, and combustion continues as the injector squirts, fuel burns and is available over a longer time. The same high compression benefits are not so pronounced with a diesel. Turbocharging a diesel does produce more power, not because of any raised compression, but because there is more air to allow a longer burn.